Figure 26-15: Nucleosynthesis in the Early Universe This graph shows how nuclei were produced between 10 seconds and 10 hours after the Big Bang. The vertical axis shows the fraction of the total mass that was in each type of particle or nucleus (p = proton, n = neutron, 2H = deuterium, 3He and 4He = helium, 6Li and 7Li = lithium, 7Be = beryllium). Very few nuclei were formed before the universe was 10 seconds old, due to the phenomenon of the deuterium bottleneck, which occurred at times earlier than those shown here. By about 103 seconds (roughly 15 minutes) after the Big Bang, the temperature had dropped below 4 × 108 K, no further nucleosynthesis was possible, and the relative amounts of different nuclei stabilized. The number of free neutrons declined rapidly as these particles decayed into protons, electrons, and antineutrinos. Agreement between these predicted mass fractions with what is observed today provides strong evidence for nuclear reactions in the early universe.
(Adapted from R. V. Wagoner)